On the reactivity of metallothioneins with 5,5′-dithiobis-(2-nitrobenzoic acid)

Abstract

Rat liver and horse kidney metallothioneins react with 5,5''-dithiobis-(2-nitrobenzoic acid) (Nbs2) to release 5-thio-2-nitrobenzoate and metal ions. The reactions are slow and exhibit biphasic kinetics with each process having an empirical rate law of the form: rate = k[RSM] .cntdot. [Nbs2] + k''[RSM], where RSM represents metal-bound thiolate groups. The pseudo-1st-order rates are insensitive to pH, but are modified in guanidine hydrochloride solution. Rat liver metallothioneins of variable Zn, Cu and Cd composition react similarly and give observable thiol/total metal ratios in good agreement with stoichiometries of SH/(Cd + Zn) of 3 and SH/Cu of 1. A model complex Cd-2,3-dimercaptopropanol, resembles the protein in its reaction with Nbs2.